Improved planar etched mirror facets

ABSTRACT

A method, and device produced therewith, for improving the planarity of etched mirror facets 18 of integrated optic structures with non-planar stripe waveguides, such as ridge or groove diode lasers or passive devices such as modulators and switches. The curvature of the mirror facet surface at the edges of the waveguide due to topographical, lithographical and etch process effects, causes detrimental phase distortions, and is avoided by widening the waveguide end near the mirror surface thereby shifting the curved facet regions away from the light mode region to surface regions where curvature is not critical.

This application is a divisional application of application Ser. No.07/533,748, filed June 6, 1990, now U.S. Pat. No. 5,031,219 of Peter L.Buchmann et al.

FIELD OF INVENTION

The invention relates to a method for improving the planarity of etchedmirror facets of integrated optic structures with non-planar stripewaveguides. Devices with active waveguides such as ridge diode lasers orgroove diode lasers, passive waveguides, as well as integrated opticmodulators and switches, are examples of structures that can be producedwith the disclosed invention.

BACKGROUND OF THE INVENTION

Integrated optic devices with non-planar stripe waveguide structure havefound wide applications in a variety of information handling systemsbecause of their compact size and because their technology is compatiblewith that of the associated electronic circuitry.

One of the most noteworthy stripe waveguide structures presently used isthe semiconductor diode laser which can advantageously be used forapplications in data communication, optical memory and laser printingsystems. In order to further improve performance, efforts are made toincrease the scale of integration of opto-electronic integratedcircuits. This requires replacing at least one cleaved mirror facet ofthe diode laser by an etched mirror. Good quality etched mirrors notonly permit integrating monitor diodes with electronic circuits, butalso facilitate such processes like mirror coating and testing at thewafer level. They also result in the added benefit of reduced handling,increased yield and decreased fabrication and testing costs. Etchedmirrors make it possible to realize very short cavity lasers,groove-coupled cavity lasers, beam deflectors and surface emitters. Inaddition, new types of lower and waveguide structures with curved andangled mirror facets can also be fabricated using etching techniques.

A laser structure which has been determined to be a good choice for highperformance applications is the so-called ridge GRINSCH (Graded-IndexSeparate Confinement Heterostructure) laser diode which is well known tothe art. The article entitled "High-Power Ridge-Waveguide AlGaAs GRINSCHLaser Diode" by Ch. Harder, et al. (Electr. Lett., Vol. 22, No. 22,Sept. 25, 1986, p. 1081-1082 discusses such a structure in great detail.It provides high quality beams at extremely low power dissipation, isvery efficient and has potential for high reliability, which is ofutmost importance for high-speed optical interconnections. The ridgewaveguide, which can be fabricated using simple and establishedprocesses, effectively stabilizes the transverse mode. GRINSCH laserdiode structures with very low threshold currents and current densitieshave been reported.

Major requirements for etching laser mirrors include: high etch rates(4-6 μm etch depth); low etch rate selectively of various materials ofthe laser structurae; vertical smooth facets with low damage; andsurface roughness less than λ/10. Chemically assisted ion beam etching(CAIBE) is known as the best method so far for the fabrication ofvertical etched mirror facets of III-V compound (e.g. AlGaAs/GaAs) laserstructures with layers of varying Al concentrations. Such processes havebeen described in various publications, which are specificallyincorporated by reference herein, namely, "Chemically Assisted Ion BeamEtching Process for High Quality Laser Mirrors" by P. Buchmann, et al.(Int. Conf. On Microlithography, Vienna, Sept. 1988), and "High PowerEtched-Facet Laser" by P. Tihanyi et al. (Electr. Lett., Vol. 23, No.15, July 16, 1987, pp. 772-773), which are incorporated by referenceherein.

An additional requirement, planarity, also referred to as flatness ofthe mirror facets, is particularly important for single-mode lasers usedin optical storage and in single-mode fiber communications. Anycurvature of the mirror surface in the light mode region causes aphase-shift distortion in the reflected and the transmitted light.Reflected light with a strong phase front distortion has a low couplingfactor to the back-travelling waveguide mode. Thus, the effectivereflectivity of a curved mirror of a single-mode waveguide is educed andthe threshhold current increases. Such phase distortions also shows upin the far-field of a laser output beam (side-lobes, multi-lobes). Anon-Gaussian beam shape of the far-field indicates that the beam cannotbe focussed to the ideal diffraction-limited spot using simple optics,resulting in a reduction of the possible storage density on, forexample, a magneto-optic disk.

Planarity of the mirror facet is difficult to achieve when the laserstructure has a pronounced topography as is the case for ridge waveguideand channeled substrate lasers, particularly where non-planar waveguidestructures are used. Two effects are primary causes of the curvature ofa mirror.

1. Topography/Lithography: Applying an etch mask layer causes someplanarization, i.e., for a ridge laser, the mask is thinner on top ofthe ridge than on the etched horizontal surfaces on both sides of it.During mask fabrication, this causes an inward recess of the mask edgewhere the ridge is positioned. This recess is transferred to the mirrorfacet by an anisotropic etching process. Although this recess rangesfrom a few tens of nm to 500 nm, it is detrimental to the laser lightwavefront (λ/2 in a GaAs laser corresponds to only 110 nm). Inexperiments, large wafer-to-wafer variations in the curvature of themirror facets were observed, with the highest distortion occurring belowthe edges of the ridge.

2. Mirror etching process: The process used for the formation of themirror facets (such as CAIBE) can also introduce a curvature of thesurface depending on the anisotropy and the undercutting of the etchingprocess. If chemically reactive gases are introduced to the etchingsystem, the local concentration of active species will vary as afunction of topography, resulting in an additional curvature of theetched facets.

This curvature problem has been recognized before and has been reportedby N. Bouadma et al., in on article "GaAs/AlGaAs Ridge Waveguide LaserMonolithically Integrated with a Photodiode Using Ion Beam Etching"(Electr. Lett., Vol. 23, No. 16, July 1987, pp. 855-857) which isspecifically incorporated by reference herein. The authors describe anattempt to solve the problem by shortening the ridge in the mirror facetregion by a few microns. With this approach, topographic effects can beavoided but at the expense of wavefront and far field distortions whichoccur because the waveguide provided by the ridge is drasticallyaffected.

Accordingly, it is a main object of the present invention to provide amethod for improving the planarity (or flatness) of etched mirror facetsof integrated optic structures with non-planar stripe waveguides.

It is another object of the invention to provide a method for etchingsemiconductor diode laser mirrors of the same or at least of similarquality than that of mirrors obtained by using conventional high-qualitycleaving techniques.

Still another object is to provide a method for etching diode lasermirrors with extremely flat, smooth and vertical surfaces using simple,easy-to-control process steps.

A further object is to provide a diode laser structure having at leastone etched mirror which is of the same or at least of similar qualitythan that of mirrors obtained by using conventional high-qualitycleaving techniques.

SUMMARY OF THE INVENTION

This invention solves the problems hitherto encountered due to curvatureof the mirror facets by using a waveguide that is widened at its endsections near the etched mirror facets. This change permits shifting thecurved facet regions away from the light mode region to the surfaceregion where curvature is not as critical.

The main advantage offered by the invention is that the problems causedby topographic and lithographic effects on the etched mirror surface areavoided, an important achievement for ridge waveguide and channeledsubstrate lasers. The resulting mirror facets, planar in the light moderegion, significantly improve the reflectivity and absorptioncoefficient of the etched mirrors as well as the far-fieldcharacteristics of single-mode etched mirror lasers. The coupling lossdue to the non-guiding widened ridge section is, on the other hand,negligible.

DESCRIPTION OF THE DRAWINGS

The invention is described in detail below with reference to drawingswhich illustrate a specific embodiment of the invention, in which:

FIG. 1 is a perspective view of a conventional ridge single quantum well(SQW) GRINSCH laser with a straight ridge resulting in curvature in thelight mode region;

FIG. 2 is a perspective view of a ridge single quantum well (SQW)GRINSCH laser with a widened-end ridge in accordance with the presentinvention, eliminating the curvature in the light mode region;

FIGS. 3A-3G are illustrations of the various steps of the inventivemirror etching method applied to fabricate a SQW GRINSCH laser withwidened ridge end sections.

DETAILED DESCRIPTION OF THE INVENTION

Prior to describing the invention in greater detail, the general conceptwill be outlined by comparing the conventional method with the inventivemethod, using a ridge AlGaAs/GaAs SQW GRINSCH structure with etchedmirrors. This example will be used in the preferred embodimentthroughout the invention.

FIG. 1 schematically illustrates the main elements of a conventionalridge GRINSCH laser structure 10. The drawing shows the structure aftercompletion of the mirror facet etching process. Deposited on a GaAssubstrate 11 are: a lower AlGaAs cladding layer 12, an active GaAs layer13 forming the quantum well, and an upper AlGaAs cladding layer 14. Thelatter has been etched to form ridge 17. The etched surfaces on bothsides of the ridge are covered with an insulation layer 15. Ametallization film 16, deposited on top of the layered laser structureprovides the electrical contact. The vertical wall 18 of an etchedmirror groove provides the mirror facet. The bottom of the groove 20displays the same profile as the top surface of the initial laserstructure, the "transferred" ridge being designated 21.

The light mode region, shown as an ellipse 24, centers around the activelayer 13 and is laterally defined by the stripe ridge 17. The shadedareas 19 illustrate the bent regions of substantial curvature in thefacet surface. These bent or curved regions cause the detrimentaldistortion laser beam wavefront, which the present invention overcomes.

FIG. 2 is a schematic illustration of a ridge SQW GRINSCH structure 22fabricated in accordance with the principles of the invention. The ridge17 is widened at its end near the mirror facet, forming a broad orflared section 23. Its width w is wider than the lateral extension ofthe light mode region 24. Thus, the bent facet regions, formed under theridge edges 19 (in FIG. 1), are shifted away from the light mode region24 to the region 29 where facet curvature is not critical. Thus, themirror surface is completely in the light mode region since it is notaffected by the cuvature in areas 29.

In order to effectively eliminate any curvature in the critical lightmode region 24, the widened waveguide section 23 needs to besubstantially wider than the lateral extension of thefull-width-at-half-maximum (FWHM) of the intensity distribution of theemitted laser beam. The length of the laterally non-guiding widenedridge section 23 has to be as short as possible in order to minimizecoupling loss between the divergent beam and the back-travellingwaveguide mode. A length of 1 to 2 μm is sufficient to avoid anytopography effects within the light mode region 24 and yet, is shortenough to keep the mode coupling loss sufficiently low so as to benegligible.

Referring now to FIGS. 3A-3G, the successive steps of the inventivemethod, applied to produce a ridge SQW GRINSCH laser, are illustrated indetail. Each figure includes two drawings, a cross-sectional view left,(denoted "-1" and a top view, denoted "-2") of the processed structure,respectively. In the preferred embodiment, the laser structure consistsof a stack of AlGaAs/GaAs layers grown on the (100) surface of an n-typeGaAs wafer using a molecular beam epitaxy (MBE) process. The mirrorfacets are provided by the vertical walls of a groove etched into thestack.

As illustrated in FIG. 3A, the process is initiated from a layeredAlGaAs/GaAs laser structure 30. It is degreased, cleaned, and made readyfor the process steps required for the fabrication of the ridge andmirror facets. For simplicity sake, only the active layer 31 of all thelayers in the stack is individually shown.

Initially, a positive resist is applied, exposed to the laser ridgepattern and developed (preferably using contact lithography and a Crmask). The patterned photoresist 32, with a (usually long) narrowsection 32.1 and a widened section 32.2, serve as a mask in the wet etchstep (in a preferred solution of H₂ SO₄ /H₂ O₂ /H₂ O) required to formthe ridge 33 and having the same shape as the photoresist. The etchedsurface is designated as 51 (FIG. 3B).

Typical dimensions for a single-mode waveguide are: ridge height of 1.5μm, ridge width of 3 μm, and ridge (or laser cavity) length of 200 to1000 μm. The widened waveguide end section must be at least 8 μm wideand about 2 μm long after mirror etching. Subsequently, a Si₃ N₄insulation layer 34, 200 nm thick is deposited, using plasma enhancedchemical vapor deposition (PECVD), thereby embedding both the ridge 33and the photoresist mask 32. The resist is then lifted off (by Acetone)leaving a self-aligned non-isolated contact stripe on top of the ridge33, and leaving etched areas 51 covered by insulation layer 34 (FIG.3C). Next, a polyimide layer is deposited and structured, using areactive ion etch (RIE) process with O₂, to serve as a lift-off mask 35for the definition of the top (p-) contact pad of the laser. The maskends about 2 μm from the widened ridge end section 33.2, as shown inFIG. 3D. A TiPtAu film is then deposited, followed by lift-off resultingin the top contact 35 extending over the length of the ridge but endingabut 2 μm in front of the widened ridge end section, thereby leaving awindow for the etching of the mirror groove (FIG. 3E).

In a next and subsequent steps, a mask 37 is formed for etching themirror groove. Whereas a single layer mask may be used, a multi-layerstructure is preferred, because it provides smoother vertical mirrorfacets. Such multi-layer etch mask and its use in the fabrication ofopto-electronic semiconductor structures is described in European PatentApplication 88.810.613.5 (filed on Sept. 12, 1988 and incorporated byreference herein. It consists of two photoresist layers, hard-bakedbottom layer and soft-baked top layer, with a thin amorphous dielectricintermediate layer sandwiched in between. The etch pattern,lithographically formed in the top resist layer, is successivelytransferred, first to the intermediate layer and then to the bottomhard-baked resist layer, the latter serving as the mask 37 during thesubsequent mirror groove etch process. It is patterned to expose theunderling semiconductor structure area 38 (FIG. 3F).

For mirror groove etching, a Cl₂ /Ar--CAIBE process is preferably usedwith an Argon energy of 500 eV and a Cl₂ flow of 15 sec/cm through afeed ring, the sample rotating at room temperature for 15 minutes. Afteretching, the hard-baked resist 37 is removed by washing in an O₂ plasma.This is followed by cleaning in solvents (FIG. 3G). The resulting mirrorgroove 39, providing mirror facet 40, has a depth of 4 to 6 μm and thefacets have a roughness less than 20 nm. Facet curvature is only foundunder the edges 41 of the widened ridge end sections, leaving the lightmode region confined to the absolutely flat region of the mirror.

FIG. 3 illustrates the fabrication of one of the mirror facets of alaser diode, although the facet at the other end of the waveguide couldalso be made at the same time. Low and high reflectivity coatings aredeposited on these front and rear facets by angled ion beam sputtering.The simplest process consists of applying an Al₂ O₃ passivation layer toeach facet. Via holes are then etched through the coating to gain accessto the bonding pads using a simple resist mask and a CF₄ --RIE step.

To simplify chip cleavage, the substrate is either lapped or wet etchedon the backside (bottom surface of the structure) to a thickness of 150μm. After Ar sputter cleaning, a GeAuNiAu metallization is evaporatedand alloyed at 390° C. for 60 seconds to form the bottom (n-) contact.

The invention herein described applies to the fabrication of a ridgediode laser, more specifically, a ridge SQW GRINSCH laser. It should,however, be understood that the invention is also applicable to otherintegrated optic structures with non-planar stripe waveguides such asgroove diode lasers (with negative topography(or passive waveguides anddevices such as modulators and switches.

It is also noted that the sequence of the process steps may be modified.In the above described embodiment, the stripe waveguide, having awidened end section, was formed before mirror facet etching. Thissequence may be changed by first etching the groove forming mirrorfacets to obtain the initial layer structure, and then forming thestripe waveguide with a widened end section.

It is evident to those skilled in the art that other materials may beadvantageously used and various other modifications made. In general,the process parameters, etchants or plasma used, the indicateddimensions and other device characteristics chosen for the descriptionof the preferred embodiment, may be changed without departing from thespirit of the invention.

What is claimed is:
 1. An integrated optical device comprising: anon-planar stripe waveguide having an etched mirror facet at at leastone end thereof, said end section being substantially wider than thelateral extension of the full-width-at-half-maximum (FWHM) of the lightmode intensity distribution of the devise such that said light modeintensity distribution is unaffected by said end section.
 2. Anintegrated optic device as in claim 1, wherein said device is a ridgediode laser comprising an active layer and surrounding cladding layersin which the light mode region is formed while in operation.
 3. Anintegrated optic device as in claim 2, wherein said widened end sectionhas a width of at least 8 μm.
 4. An integrated optic device as in claim2, wherein said widened end section has a length between 1 and 5 μm. 5.An integrated optic device as in claim 1, wherein said widened endsection has a width of at least 8 μm.
 6. An integrated optic device asin claim 1, wherein said widened end section has a length between 1 and5 μm.